1. Field of the Invention
The present invention relates to an autonomous distributed field bus system applied to an industrial measuring control system. More particularly, the present invention relates to an autonomous distributed field bus system which can maintain normal operation stably when a failure occurs in a host control apparatus and which enables trial operation needed for the construction of a new system to be readily performed in another aspect.
2. Description of the Related Art
A typical configuration of conventional field bus systems will be explained with reference to FIG. 5. The field bus system consists of a control room and a field. The control room has a host apparatus 51 for controlling/managing the entire system. The field contains a plurality of blocks 52 for executing various detections and control operations. Each of the blocks 52 and the host apparatus 51 make a control loop in their respective combination. The host apparatus 51 and the individual blocks 52 are interconnected by a field bus 53. Multidrop connection is used for the connection between the host apparatus 51 and the blocks 52. The host apparatus 51 includes an arithmetic and control device 51a, a communication and bus control device 51b and a power device 51c. Each of the blocks 52 includes a transmitter 54 acting as a detection device, and an actuator 55 acting as an operation device. Every block 52 is connected to the field bus 53 through a junction device 56.
In the system arranged in the manner mentioned above, a detection signal output from the transmitter 54 is sent to the host apparatus 51 through the field bus 53. The arithmetic and control device 51a produces a control signal on the basis of the data obtained from the detection signal. The control signal is sent through the field bus 53 to the actuator 55 to control the actuator 55 in a desired manner. When necessary, the actuator 55 returns an answer back signal to the host apparatus 51 through the field bus 53.
The aforementioned transmission operations between each of the many blocks 52 and the host apparatus 51 are adequately timed. Therefore, various types of data from the host apparatus 51 and the individual blocks 52 flow on the field bus 53. The flow of data on the field bus 53 is controlled by the communication and bus control device 51b of the host apparatus 51. Thus, the communication and bus control device 51b adjusts communication requests between the components of the system.
According to the configuration of the conventional field bus system, communications and controls are performed by the host apparatus 51 which adopts a digital distributed control system (DCS). Control of the flow of signals in the entire field bus and management as well as control of the bus line are performed by the digital distributed control system. A configuration of the digital distributed control system exists as an extension of the conventional measuring control system and is hence natural as a system configuration.
However, such a conventional system has the following problems.
First, the host apparatus of the control room participates in the calculations and controls required for controlling the control loop, including the detection device and the operation device. It is therefore likely that any failure in the host apparatus will lead to a failure of the entire system. In other words, the conventional system is susceptible to damage by failures in the control room.
Secondly, when the conventional system is newly constructed, first, individual local devices are installed and their operations are checked, and then the host apparatus is installed and connected to the local devices and the operation of the entire system is checked. Since progress of construction of the field instruments, which are the local devices, in general differs, checking of the operation of the combination of these field instruments and the host apparatus, which is done after checking all of the operations of the field instruments, is delayed considerably. Before the field instruments are respectively connected to the host apparatus, they must be run on a trial basis without participation of the host apparatus. This is done by using a simulator in place of the host apparatus. This simulator is controlled on a real-time basis and is quite large in scale. In addition, excellent operation of the field instruments obtained by using the simulator does not guarantee excellent actual operation, that is, perfect and safe operation of the field instruments.
Thirdly, control of the entire system is disabled by failures occurring in the host apparatus, or disconnection of the communication path between the host apparatus in the control room and the junction device in the field due to noises, that is, the system is susceptible to damage by the failures. In the above-mentioned conventional system, trouble readily occurs, particularly in the communication path between the host apparatus and the junction device.
Fourthly, in a steady state of control, the host apparatus must exchange data with the detection device and the operation device of each of the control loops at the response speed required for each of the control loops. Since some of the control loops may have a slow response speed, the host apparatus's communication load of the communications with such control loops increases, thus increasing the entire communication load of the host apparatus.